1. Field of the Invention
The present invention relates to a light guide plate used for a planar light source that illuminates a liquid crystal panel from its back and more particularly to a light guide plate for a planar light source suitably applied to small liquid crystal panels used on cell phones.
2. Description of Related Art
A growing number of thin, easy-to-see liquid crystal displays having a backlight mechanism are being used as displays on small, thin information devices, such as notebook word processors or computers, cell phones and portable TV sets. Such a backlight mechanism uses a planar light source to illuminate an entire surface or a liquid crystal panel from its back. The planar light source generally comprises a light source, such as a fluorescent lamp or a light emitting diode (LED), and a light guide plate that converts a light flux into a planar light flux for illuminating the liquid crystal panel. As for the light source, an increasing number of planar light sources are using LEDs as the light source for further reductions in size and thickness and for increased longevity. These planar light sources may be classified into a direct type, in which the light source is arranged directly below the light guide plate, and a side light type, in which the light source is arranged at the sides of the light guide plate. For devices that put importance on small size and small thickness, such as cell phones, the side light type is usually adopted.
Now, a conventional side light type planar light source will be explained by referring to FIG. 1 and FIG. 2. FIG. 1 shows an example of a conventional planar light source of this kind. It basically consists of a rectangular prism-shaped light guide plate 1 made of a transparent material and a light source 2 having three LEDs arranged at the side of the light guide plate 1.
The light guide plate 1 is arranged on the back of a liquid crystal panel 7. The light guide plate 1 is often provided on its underside with a light reflection sheet 8 that directs light from the light source 2 toward the liquid crystal panel 7, as shown in FIG. 2B. On the upper surface side of the light guide plate 1 there are provided a diffusion sheet 9 that uniformly scatters light from the light source 2 and a prism sheet 10 that focuses light toward the liquid crystal panel 7.
The light guide plate 1 is a rectangular prism-shaped plate member capable of transmitting light, which is formed of, for example, a colorless, transparent plastic material. The upper surface of the light guide plate 1 is used as a light emitting face 1b and one of side surfaces of the light guide plate 1 is used as a light incidence face 1a. The light source 2 is arranged at a position facing the light incidence face 1a. A light beam 3 radiated from the light source 2 enters the light incidence face 1a and then is repetitively reflected in the light guide plate toward the light emitting face 1b as it travels in the light guide plate until those components of light incident on the upper surface at smaller than the critical angle are extracted from the light emitting face 1b of the light guide plate 1 as illumination light 4. The illumination light 4 extracted outside then illuminates the liquid crystal panel 7 from the back.
FIG. 2A and FIG. 2B show another example of a conventional side light type planar light source (see Japanese Patent Disclosure No. 2003-262734, page 2 and FIG. 3). In the planar light source of this kind, one corner portion 1d of the almost rectangular prism-shaped light guide plate 1 is cut off to form an additional side surface as the light incidence face 1a. A light source 2 made up of one LED is arranged at a position facing the light incidence face 1a. As shown in FIG. 2B, the upper surface of the light guide plate 1 constitutes a light emitting face 1b, and a bottom surface 1c opposite the light emitting face 1b is used as a light reflection surface that is formed with a fine texture or a plurality of hemispherical dots to reflect the incoming light 5 toward the light emitting face 1b. 
In the above conventional side light type planar light sources when the incident light 3 from the light source 2 enters the light guide plate 1 at an incidence angle a, as shown in FIG. 3A and FIG. 3B, the light is refracted and travels in the light guide plate 1 at an angle b with respect to a normal, as indicated at 5. Since the material of the light guide plate 1, such as acrylic resin and polycarbonate resin, has a higher refractive index than that of air, the angle b with respect to the normal is smaller than the incidence angle a. At this time, the incident light 3 from the light source 2 has a directivity of the LED itself, so the directivity or the light 5, which is refracted after it has entered the light guide plate 1, is narrower than that of the incident light 3.
FIG. 4 shows directivities of the light 3 from the light source 2 and of the light 5 after it enters the light guide plate 1. FIG. 4A shows a directivity of the incident light 3 from an LED as the light source 2 and FIG. 4B shows a directivity of the incoming light S that has entered the light guide plate 1 from the light incidence face 1a. The light beam 3 of the LED as a point light source has a directivity indicated by a curve 101 in FIG. 4A. The light 5, which has resulted from the light 3 entering the light guide plate 1, has a directivity indicated by a curve 102 in FIG. 4B. As described above, the directivity of the light 5 after it has entered the light guide plate 1 is narrower than that of the incident light 3 of the LED itself. Thus, in the conventional light guide plate 1 which has the light incidence face 1a formed as a flat surface, there is a problem that a distribution of intensity of the light 5 after it enters the light guide plate 1 is ununiform.
To solve this problem, a light guide plate has been proposed in which a light incidence face of the light guide plate is provided with undulations made by a plurality of prisms of similar shape (see Japanese Patent Disclosure No. 2002-196151, pages 3-5 and FIG. 2). A light guide plate 11 shown in FIG. 5 and FIG. 6 has the similar construction to that of the conventional light guide plate 1 except that a light incidence face 11a differs in shape from the counterpart of the conventional light guide plate 1. So, only the construction of the light incidence face 11a will be explained and descriptions of other constructions omitted. This construction similarly applies also to other conventional light guide plates whose corners are cut off, so in the following explanation we take the light guide plate 11 of FIG. 5 and FIG. 6 as a representative conventional light guide plate.
As shown in FIG. 5A and FIG. 5B, the light guide plate 11 has a light incidence face 11a on one side which forms into undulations. The undulated surface portion has a uniform distribution of prismlike protrusions 12. The prismlike protrusions 12 each have a triangular shape in cross section defined by a pair of inclined surfaces 12a, 12b. Between the adjacent protrusions 12 there is a flat portion 13.
When the light incidence face 11a of the light guide plate 11 is taken as a virtual plane, the angle that light beams 15, 16 make with the normal after the beams have entered into the light guide plate 11 can be made larger than the angle they make in the case of the light guide plate 1, by the effect of the inclined surfaces 12a, 12b of the prismlike protrusions 12 on the light incidence face 11a, as shown in FIG. 6. This is true even for light beams whose incidence angle on the light incidence face 11a is large. Thus, when the light incidence face 11a is seen as a whole, a range of angle of the light beams 15, 17 that have entered into the light guide plate 11 from the prismlike protrusions 12 can be increased. Further, the light beam 16 that has entered the light guide plate 11 from the flat portion 13 enters straight into the light guide plate 11, as in the case of the planar light incidence face 11a of the light guide plate 1.
However, since in the light guide plate 11 the light incidence face 11a formed by the prismlike protrusions 12 and the flat portion 12 is discontinuous in shape, as shown in FIG. 6, the directivities of light beams 15, 16, 17 that have entered into the light guide plate 11 from the light incidence face 11a are distorted as shown in FIG. 7. The light beam 16 that enters the light guide plate 11 from the flat portion 13 of the light guide plate 11 in FIG. 6 has a directivity indicated by a curve 104 in FIG. 7. The light beam 17 that enters the light guide plate 11 from one 12a of the inclined surfaces 12 of the light guide plate 11 in FIG. 6 has a directivity indicated by a curve 105 in FIG. 7. The light beam 15 that enters the light guide plate 11 from the other 12b of the prismlike protrusions 12 of the light guide plate 11 in FIG. 6 has a directivity indicated by a curve 103 in FIG. 7.
In the conventional light guide plate 11 as described above, the light beams 15, 16, 17 that enters the light guide plate 11 from the light incidence face 11a have distorted directivities, giving rise to a problem that the light intensity distribution becomes non-uniform because of the distorted characteristics of the directivities If the light guide plate 11 with an ununiform light intensity distribution is used in a side light type planar light source for a liquid crystal display, there is a problem that bright lines are produced from the light source or the brightness on the display screen varies, significantly degrading an image quality.